Elasticity in Ferromagnetic Shape Memory Alloys

Abstract

Ferromagnetic shape memory alloys (FSMAs) are a new class of active materials, which combine the properties of ferromagnetism with those of a diffusionless, reversible martensitic transformation. These materials are technologically interesting due to the possibility of inducing large shape changes with an external applied magnetic field; either inducing the austenite/ martensite transformation or rearranging the martensitic variant structure with an applied field will induce a reversible shape change. The dependence of a solid's elastic properties on temperature in the vicinity of a structural transformation provides insight into the nature of the transition. Therefore, the elasticity of Ni2MnGa and Fe3Pd were studied.

The temperature dependence of the elastic constants of the austenitic Ni0.50Mn0.284Ga0.216 and Ni0.49Mn0.234Ga0.276 were studied by an ultrasonic continuous wave method. Anomalous behavior in austenite was observed, which indicates a premartensitic transition. The temperature dependence of the elastic constants in martensitic Ni0.50Mn0.284Ga0.216 indicates a structural phase change from the tetragonal to a second phase at lower temperature. Modeling this phase change as a reentrance transition reproduces the major aspects of the temperature dependence of the shear elastic constant, (C11-C12)/2.

The elasticity as a function of temperature and magnetic field of Fe3Pd was studied as well. An abrupt change of the elastic constants at around 45OC indicates a possible premartensitic transformation. The magnetic field dependence of elastic constants also indicates a probably magnetic field induced transition.